Method and apparatus for detecting and treating vulnerable plaques

ABSTRACT

Apparatus for detecting vulnerable plaques embedded in the wall of a patient&#39;s blood vessel includes an intravascular catheter containing a microwave antenna, an extra-corporeal radiometer having a signal input, a reference input and an output, a cable for electrically connecting the antenna to the signal input, and a device for applying an indication of the patient&#39;s normal tissue temperature to the reference input so that when the catheter is moved along the vessel, the locations of the vulnerable plaques are reflected in a signal from the output as thermal anomalies due to the higher emissivity of the vulnerable plaques as compared to the normal tissue. A second embodiment of the apparatus has two coaxial antennas in the catheter serving two radiometers. One measures the temperature at locations in the vessel wall, the other measures the temperature at the surface. By subtracting the two signals, the locations of vulnerable plaque may be visualized. The apparatus employs a special diplexer for separating the signals from the two antennas and a method of detecting and possibly destroying the plaques is also disclosed.

[0001] This invention relates to a minimally invasive technique fordetecting vulnerable plaques. It relates especially to method andapparatus for detecting vulnerable plaques utilizing microwaveradiometry.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] It is widely known that many heart attacks originate fromblockages created by athrosclerosis which is the aggressive accumulationof plaques in the coronary arteries. The accumulation of lipids in theartery and resulting tissue reaction cause a narrowing of the affectedartery which can result in angina, coronary occlusion and even cardiacdeath.

[0004] Relatively recent studies have shown that coronary disease canalso be caused by so-called vulnerable plaques which, unlike occlusiveplaque, are engrained or imbedded in the arterial wall and do not growinto the blood vessel. Rather, they tend to erode creating a raw tissuesurface that forms caps or scabs. Thus, they are more dangerous thanoccluding plaque which usually give a warning to a patient in the formof pain or shortness of breath. See, e.g., The Coming of Age ofVulnerable Plaque, Diller, W., Windover's Review of Emerging MedicalVentures, November 2000.

[0005] 2. Description of the Prior Art

[0006] Since vulnerable plaques are contained within the vessel wall,they do not result in a closing or narrowing of that vessel. As aresult, such plaques are not easily detectable using conventional x-ray,ultrasound and MRI imaging techniques.

[0007] Moreover, since vulnerable plaques are part of the vessel wall,they may have essentially the same temperature as the surrounding normaltissue and the blood flowing through the vessel. Therefore, they are notamenable to detection by known intravascular catheters which rely oninfrared (IR) imaging, thermisters, thermocouples and the like in orderto detect temperature differences in the vessel wall.

[0008] Such intravascular catheters are disadvantaged also because theyusually incorporate an inflatable balloon to isolate the working eachend of the catheter from fluids in the vessel; see for example Pat. No.6,475,159. As seen there, the IR detector is located within the balloonwhich constitutes an insulating (not transparent at IR frequencies)layer between the detector and the vessel wall causing significantattenuation of the signal from the detector. Also, the undesirablestoppage of blood flow by the balloon increases the risk to the patient.Still further, the balloon has to be repeatedly inflated and deflated inorder to image different locations along the blood vessel increasing theoperating time during which the patient is at risk.

SUMMARY OF THE INVENTION

[0009] Accordingly, it is an object of the present invention to providea method of detecting vulnerable plaques before the plaques rupture andcause thrombosis.

[0010] Another object of the invention is to provide such a vulnerableplaque detection method which does not require the stoppage of bloodflow in the affected vessel.

[0011] An additional object of the invention is to provide a method ofdetecting vulnerable plaques using microwave radiometry.

[0012] Another object of the invention is to provide intracorporealmicrowave apparatus for detecting vulnerable plaques having one or moreof the above advantages.

[0013] A further object of the invention is to provide such apparatuscapable of treating the plaques after detection.

[0014] Other objects will, in part, be obvious and will, in part, appearhereinafter.

[0015] The invention accordingly comprises the several steps and therelation of one or more of such steps with respect to each of theothers, and the apparatus embodying the features of construction,combination of elements and arrangement of parts which are adapted toeffect such steps, all as exemplified in the following detaileddescription, and the scope of the invention will be indicated in theclaims.

[0016] Briefly, the present method utilizes microwave radiometry todetect the presence of vulnerable plaques engrained in the wall of ablood vessel. In accordance with the method, an intravascular cathetercontaining at least one microwave antenna is moved along the suspectvessel. The antenna, in combination with an external microwave detectionand display unit, is able to detect and display thermal anomalies due tothe difference in the thermal emissivity (brightness) of vulnerableplaques as compared to normal tissue even though the two may have acommon temperature. In other words, it has been found that the microwavecharacteristics of vulnerable plaques imbedded in a vessel wall aredifferent from those of normal tissue comprising the vessel wall andthis difference is detected as a thermal anomaly and displayed orplotted as the catheter is moved along the vessel.

[0017] As we shall see, in some applications the detected plaques maythen by treated by microwave ablation using the very same catheter.

[0018] In its simplest form, the microwave antenna may be a more or lessconventional microwave antenna located at the distal or working end ofthe catheter. The inner and outer conductors of the antenna areconnected by a coaxial cable to an external detection and display unitwhich detects the microwave emissions from the blood vessel picked up bythe antenna and produces corresponding output signals for a displaywhich responds to those signals by displaying the thermal emissions fromthe blood vessel in real time as the catheter is moved along the vessel.

[0019] In accordance with the invention, the radiometer is preferably aDicke switch-type radiometer and the temperature of the blood flowingthrough the vessel, which corresponds to the body's core temperature, isused as the radiometer reference. The operating frequency of theradiometer is selected to detect microwave emissions from a depth in theblood vessel wall where vulnerable plaques are likely to be imbedded,e.g. a frequency in the range of 1 to 4 GHz, preferably 1 GHz. Thus asthe catheter is moved along the vessel, it is maintained at a constantbackground or core temperature corresponding to the temperature of theblood and of normal tissue. The locations of vulnerable plaques aredetected as thermal anomalies (hot spots) due to the higher emissivityof the plaques as compared to normal tissue. Using the output of theradiometer to control a display, the plaque sites along the vessel canbe plotted.

[0020] It is important to note that the present method and apparatusallow the detection of vulnerable plaques at subsurface locations in thevessel wall without contacting the vessel wall and without anyinterruption of blood flow through the vessel.

[0021] As will be described in more detail later, the catheter mayinclude a lengthwise passage for receiving a guide wire to help guidethe catheter into and along the blood vessel being examined. As we shallsee, in some applications the guide wire itself may actually constitutethe inner conductor of the antenna within the catheter. Also, in orderto help center the antenna within the blood vessel, the catheter mayincorporate an expandable perforated standoff device which spaces theantenna from the vessel wall without materially interfering with theblood flow through the vessel.

[0022] In a preferred embodiment of the invention, the detection unitincludes two radiometers operating at different frequencies. Oneradiometer, operating at a higher frequency in the range of 3 to 6 GHz,preferably 4 GHz, detects thermal emissions from the inner surface ofthe blood vessel. This temperature, corresponding to the body coretemperature, is used as a reference. The second radiometer operates at alower frequency of 1 to 4 GHz, preferably 1 GHz, to detect thermalemissions from subsurface locations in the vessel wall which may containembedded vulnerable plaques. Thus by subtracting the outputs of the tworadiometers, the sites of vulnerable plaques can be detected anddisplayed continuously and in real time as the catheter is moved alongthe blood vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For a fuller understanding of the nature and objects of theinvention, reference should be made to the following detaileddescription taken in connection with the accompanying drawings, inwhich:

[0024]FIG. 1 is a diagrammatic view of apparatus for detectingvulnerable plaques in accordance with the invention and employing afirst intravascular catheter embodiment;

[0025]FIG. 2 is a fragmentary sectional view of a second catheterembodiment for use with the FIG. 1 apparatus;

[0026]FIG. 3 is a similar view showing a third catheter embodiment;

[0027]FIG. 4 is a view similar to FIG. 1 of another apparatusembodiment, and

[0028]FIG. 5 is a sectional view of the diplexer component of theapparatus.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0029] Referring to FIG. 1 of the drawings, the present apparatuscomprises a catheter shown generally at 10 for insertion into a bloodvessel V which may have locations where vulnerable plaques P areembedded or engrained in the vessel wall. Such plaques P typicallyinclude a relatively large portion of the vessel wall, e.g. a third to ahalf of its circumference. Catheter 10 is connected by coaxial cables 12a and 12 b to a detection and display unit 14. The catheter has aproximal end 10 a to which cables 12 a and 12 b are connected by way ofa fitting or connector 16 and a distal end or tip 10 b. The catheter mayhave a length of 100 cm or more and is quite narrow and flexible so thatit can be threaded along a conventional introducer, e.g. 8.5 French,allowing the distal end 10 b of the catheter to be placed at a selectedposition in a patient's blood vessel V. Typically, vessel V is accessedby a vein in the patient's neck or groin.

[0030] The catheter may include an expandable perforated stand-offdevice such as shown Pat. No. 6,496,738, which is hereby incorporatedherein by reference, so as to center the catheter 10 in vessel V withoutimpeding the blood flow through that vessel.

[0031] As shown in FIG. 1, catheter 10 comprises a central conductor 22surrounded by a cylindrical layer 24 of a suitable low loss dielectricmaterial. Surrounding the layer 24 is a tubular middle conductor 26surrounded by a dielectric layer 28. Finally, a tubular outer conductor30 encircles layer 28. At fitting 16, the proximal ends of conductors22, 26 and 30 are connected by way of a passive diplexer 31 (FIG. 5) tothe coaxial cables 12 a and 12 b. Preferably, the catheter has aprotective outer coating, e.g. of PTFE, (not shown).

[0032] At the distal end segment of catheter 10, the middle conductor 26extends beyond the outer conductor 30 to form a microwave antenna 32which, in this case, is a monopole as in the above Pat. No. 6,496,738.In some applications, a helical antenna or capacitive tip may be used;see Pat. Nos. 4,583,556 and 4,557,272, the contents of which are herebyincorporated herein by reference.

[0033] The distal end 10 b of the catheter is actually formed by arounded low loss dielectric button 34 which is butted and secured to thedistal end of the dielectric layers 24 and 28. Typically, the middleconductor 26 extends beyond the outer conductor 30 a distance in theorder of 1 cm so that antenna 32 has a relatively long antenna pattern.Also, if desired, the diameters of the coaxial conductors in catheter 10may be stepped down along the catheter to improve antenna performance.Antenna 32 detects the thermal radiation emitted from blood vessel V andapplies a corresponding electrical signal via cable 12 a to a radiometer38 in unit 14. The radiometer 38 may be a conventional Dicke switch-typeradiometer as described in the above Pat. No. 4,557,272. Thetemperature-indicating signal from antenna 32 is applied via cable 12 ato the signal input 40 a of the Dicke switch 40 in radiometer 38. Theother input to the switch 40 is a reference value which corresponds tothe patient's core temperature, e.g. 37° C.

[0034] That temperature may be measured using a resistive termination orload or heat sensor 42 connected between inner conductor 22 and middleconductor 26 near the catheter tip. The sensor output or value isapplied via those conductors to diplexer 31 in connector 16 whichseparates that signal from the antenna signal. That reference signal isthereupon conducted by cable 12 b to the reference input 40 b of switch40. In other words, two ports of the radiometer are brought out toreceive both the antenna and reference signals from catheter 10. Theadvantage of this arrangement is that the unknown temperature is nowcompared with the actual blood (core) temperature. This improvesradiometer sensitivity (performance) by keeping all circuitry thatprecedes the Dicke switch at the same temperature.

[0035] The radiometer operates at a center frequency in the order of 1to 4 GHz so that the apparatus can detect thermal emissions fromlocations relatively deep in the wall of vessel V.

[0036] The output of the radiometer 38 is processed by a processor 44 inunit 14 which controls a display 46.

[0037] To use the FIG. 1 apparatus, the catheter 10 is threaded into thepatient's vessel V in the usual way. After insertion, the catheterassumes essentially the same temperature as the vessel V and the bloodflowing through the vessel. This temperature as sensed by sensor 42 isused as the reference for Dicke switch 40 which toggles between itssignal and reference inputs 40 a and 40 b in the usual way. When thecatheter is moved along the vessel V, say, in the direction of the arrowA, the antenna 32 will pick up thermal emissions from the normal tissuein the vessel wall and unit 14 will provide a core or backgroundtemperature indication which will be displayed by display 46. However,when the antenna 34 is moved opposite a region containing vulnerableplaques P, the apparatus will detect a thermal anomaly due to theincreased emittance (brightness) of the plaques embedded in the vesselwall. Thus as the catheter 10 is moved along the vessel, the unit 14 candisplay continuously in real time the locations of plaques P as well asother useful information such as the body's core temperature, diagnosticdata and the like as instructed via the processor's keyboard 44 a.

[0038] Referring now to FIG. 2, in some procedures, it may be desirablethat the catheter be guided along the blood vessel V by means of a guidewire. FIG. 2 illustrates an intravascular catheter shown generally at 50capable of being moved along a guide wire W previously introduced intothe blood vessel in a conventional manner. Catheter 50 is similar tocatheter 10 in FIG. 1 except that its central conductor 54 is anelongated flexible conductive tube. The other parts of catheter 50 aremore or less the same as those of catheter 10 and therefore carry thesame identifying numerals.

[0039] In catheter 50, conductor 54 extends through the fitting 16 aswell as all the way through the button 34 to the tip 10 b of thecatheter. This allows the guide wire W to be threaded through thetubular conductor 54 so that the catheter 50 can be moved along theguide wire after the guide wire has been introduced into the bloodvessel being examined.

[0040] When the catheter 50 is in use, the guide wire W does notinterfere with the antenna pattern of antenna 32 because it is shieldedby conductor 54. In other words, the field around the antenna does notextend within the metal conductor 54.

[0041] In some applications, the guide wire W itself may be used as thecentral conductor of the antenna 32 in the catheter. FIG. 3 shows such acatheter at 60 which may be used to detect vulnerable plaques asdescribed above. As shown there, the catheter 60 is similar to catheter50 except that it is devoid of the tubular central conductor 54.Instead, it is formed with an axial passage 62 in dielectric layer 24and button 34 which passage extends snugly but slidably all the way fromthe tip of the catheter to the proximal end thereof and through thefitting 16 so that the guide wire W can be threaded through passage 62as shown. In this case, the guide wire itself is connected electricallyvia cable 12 b to the detection and display unit 14. In use, the guidewire may be introduced into the blood vessel to be examined and thenremain stationary while the remainder of the catheter is slid along theguide wire in order to examine different lengthwise segments of theblood vessel wall. Alternatively, the entire catheter 60 including theguide wire W may be moved as a unit along the blood vessel in order toadvance the antenna 32 along that vessel.

[0042] Refer now to FIG. 4 which shows a preferred embodiment of theinvention that can detect even very small thermal anomalies in thevessel wall due to embedded or engrained plaques. The FIG. 4 apparatuscomprises a catheter shown generally at 70 having coaxial inner andouter antennas indicated at 72 a and 72 b. The inner antenna 72 acomprises an inner conductor 74 and an outer conductor 76 separated byan insulating layer 78. The inner conductor 74 extends beyond the outerconductor 76 forming the antenna 72 a.

[0043] The outer antenna 72 b comprises a tubular inner conductor 82 andan outer conductor 84 separated by an insulating layer 86, the innerconductor 72 extending beyond the outer conductor 74 to form the antenna72 b. The proximate end of catheter 70 is terminated by a fitting orconnector 88 containing a diplexer 31 (FIG. 5) which connects theconductors of the antennas 72 a, 72 b to coaxial cables 90 a and 90 bleading to a detection and display unit 92.

[0044] In some applications, the outer conductor 76 of antenna 72 a andthe inner conductor 82 of antenna 72 b may be a common conductor. Morepreferably those conductors are separate as shown so that the innerantenna 72 a is slidable within the outer antenna 72 b and fitting 88 sothat the distance D between the two antennas can be varied from zero toseveral centimeters allowing the outer and inner antennas to beoptimized at two specific frequencies F₁ and F₂. The inner conductor 74of antenna 72 a may be hollow or tubular so that it can receive a guidewire as described in connection with FIG. 2. Alternatively, thatconductor may be sufficiently small to serve as the guide wire itself asdescribed in connection with FIG. 3.

[0045] In order to electrically separate the outputs of the two antennas72 a and 72 b, the fitting or connector 88, like connector 16,incorporates a passive diplexer 31. As seen from FIG. 5, the diplexerincludes a quarter-wave (λ/4) stub 91 to bring out the signal F₂ fromthe inner antenna 72 a. This stub also provides a matched 90° bend toseparate and bring out the signal F₁ from the outer antenna 72 b.

[0046] Whereas it is known in the art to use a quarter-wave stub tosupport the center conductor of an antenna, the present diplexer has atubular inner conductor 94 which receives the coaxial cable 74-78comprising the inner antenna 72 a providing signal F₂. That conductor 94may be an extension of the antenna conductor 82. Surrounding andinsulated from conductor 94 is a coaxial outer conductor 96 which may bean extension of antenna conductor 84. The two diplexer conductors 94 and96 are shorted by an end plate 98 at the end of stub 91. Conductor 94has a branch 94 a which is brought out through a branch 96 a ofconductor 96 to deliver the signal F₁ from antenna 72 b. Preferably, thecoaxial cable 74-78 is slidable to some extent along conductor 94 tovary the antenna distance D as described above.

[0047] The illustrated diplexer 31 provides several distinct advantages.It separates the concentric cables from antennas 72 a and 72 b in FIG. 4into two separate cables; allows those cables to be mechanically andindependently positioned, and it allows the innermost conductor todouble as a guide wire for the catheter as shown in FIGS. 2 and 3.

[0048] In the FIG. 4 apparatus, the smaller diameter antenna 72 a,optimized at a frequency F₂, e.g. 3-6 GHz, may measure the blood andnormal tissue temperature, whereas the larger diameter antenna 72 boptimized at frequency F₂, e.g. 1-4 GHz, measures the temperature of thedeeper tissue where vulnerable plaques are likely to occur. This largerdiameter provides a less lossy cable making that antenna more efficient.The larger diameter antenna also places it closer to the wall of vesselV (FIG. 1), further increasing the depth of detection.

[0049] While the catheter 70 in FIG. 4 could be connected by cables 90 aand 90 b to the switch 40 of a single radiometer as shown in FIG. 1, theillustrated detection and display unit 92 contains two radiometers 38 aand 38 b connects to cables 90 a and 90 b, respectively. The formerwhich operates at the frequency F₁ detects thermal anomalies picked upby antenna 72 b due to plaques located relatively deep in the wall ofvessel V (FIG. 1) as before; the latter operating at frequency F₂detects thermal emissions picked up by antenna 72 a at the inner surfaceof the vessel which reflect the body core temperature. The processor 44thereupon subtracts the signals and causes display 46 to display thelocations of thermal anomalies which are likely due to plaques P.

[0050] After the vessel V has been examined and the locations of theplaques P determined as described above, the plaques P may be treated bymicrowave ablation using the very same catheter. This may be done byconnecting the catheter via a diplexer to a microwave transmitter inorder to heat the plaques as described in the above Pat. No. 6,496,738.

[0051] It will thus be seen that the objects set forth above, amongthose made apparent from the preceding description, are efficientlyattained and, since certain changes may be made in carrying out theabove method and in the constructions set forth without departing fromthe scope of the invention, it is intended that all matter contained inthe above description or shown in the accompanying drawings shall beinterpreted as illustrative and not in the limiting sense.

[0052] Is also to be understood that the following claims are intendedto cover all of the generic and specific features of the inventiondescribed herein.

What is claimed is:
 1. Apparatus for detecting vulnerable plaquesembedded or engrained in the wall of a patient's blood vessel, saidapparatus comprising an intravascular catheter containing a microwaveantenna; a radiometer having a signal input, a reference input and anoutput; a diplexer electrically connecting the antenna to the signalinput, and means for applying an indication of the patient's normaltissue temperature to the diplexer which routes said indication to thereference input so that when the catheter is moved along said vessel,the locations of said vulnerable plaques are reflected in a signal fromsaid output as thermal anomalies due to the higher emissivity of saidvulnerable plaques as compared to normal tissue.
 2. The apparatusdefined in claim 1 wherein the antenna comprises a coaxial cableextending lengthwise along the catheter.
 3. The apparatus defined inclaim 2 wherein the coaxial cable is stepped along its length tooptimize its performance.
 4. The apparatus defined in claim 2 whereinthe coaxial cable has an outer conductor and an inner conductorelectrically insulated from the outer conductor and which extendslengthwise beyond the outer conductor.
 5. The apparatus defined in claim4 wherein the inner conductor is an open-ended tube which extends thelength of the catheter and is adapted to receive a guide wire.
 6. Theapparatus defined in claim 5 and further including a guide wire slidablyreceived in said tube for guiding the catheter along said blood vessel.7. The apparatus defined in claim 4 wherein the inner conductor consistsof a conductive wire slidably received within and insulated from theouter conductor, said wire constituting both the inner conductor of saidantenna and a guide wire for the catheter.
 8. The apparatus defined inclaim 1 wherein said radiometer has an operating frequency in the rangeof 1 to 4 GHz.
 9. The apparatus defined in claim 1 and further includingdisplay means responsive to the output signal from the radiometer fordisplaying the locations of said thermal anomalies along said bloodvessel.
 10. The apparatus defined in claim 1 wherein said means forapplying comprise temperature sensing means in the catheter, and a cableconnecting the output of the sensing means to said diplexer.
 11. Theapparatus defined in claim 10 wherein said temperature sensing meansinclude an impedance in said catheter.
 12. Apparatus for detectingvulnerable plaques embedded or ingrained in the wall of a patient'sblood vessel, said apparatus comprising an intravascular cathetercontaining a microwave antenna and a temperature sensor; a radiometerhaving a signal input, a reference input and an output and operating ata selected frequency; a diplexer connecting the antenna and sensor tothe signal and reference inputs, respectively, of said radiometer sothat when the catheter is moved along said blood vessel, said radiometeroutput delivers a plaque-indicating output signal, and means responsiveto the output signal for displaying thermal anomalies due to the higheremissivity of said vulnerable plaques as compared to normal tissue. 13.The apparatus defined in claim 12 wherein the antenna comprises acoaxial cable composed of electrically isolated inner and outerconductors extending lengthwise within the catheter, said innerconductor extending beyond the outer conductor.
 14. The apparatusdefined in claim 13 wherein the inner conductor is an open-ended tube.15. The apparatus defined in claim 14 and further including a guide wireslidably received in said tube for guiding the catheter along said bloodvessel.
 16. The apparatus defined in claim 12 wherein the innerconductor consists of a conductive wire slidably received within theouter conductor, said wire also constituting a guide wire for thecatheter.
 17. Apparatus for detecting vulnerable plaques embedded oringrained in the wall of the patient's blood vessel, said apparatuscomprising an intravascular catheter having proximal and distal ends andcontaining an inner coaxial cable forming a first antenna and an outercoaxial cable forming a second antenna, said first antenna extendinglengthwise beyond the second antenna a selected distance, and said firstand second antennas being dimensioned to pick up different first andsecond signal frequencies, respectively, and a diplexer at the proximalend of the catheter, said diplexer having a tubular inner conductor anda coaxial outer conductor, said inner conductor receiving the innercoaxial cable and the inner and outer conductors being connectedelectrically to the outer coaxial cable, said outer conductor having abranch oriented 90° relative to the inner conductor and spaced fromcorresponding shorted ends of the inner and outer conductors to form aquarter wave stub at the second signal frequency.
 18. The apparatusdefined in claim 17 wherein the inner coaxial cable is slidablelengthwise relative to the outer antenna and diplexer so as to adjustsaid selected distance.
 19. The apparatus defined in claim 17 whereinthe inner coaxial cable has an open-ended tubular inner conductor whichextends the length of the catheter and through the diplexer.
 20. Theapparatus defined in claim 19 and further including a guide wireslidably received in the inner conductor and diplexer for guiding thecatheter along said blood vessel.
 21. The apparatus defined in claim 17wherein the inner coaxial cable includes an axial wire that is slidablerelative to the outer coaxial cable and diplexer, said wire constitutingboth the first conductor of said inner antenna and a guide wire for thecatheter.
 22. The apparatus defined in claim 17 and further including afirst radiometer connected to the inner coaxial cable and tuned todetect signals picked up by the first antenna and produce a first outputsignal; a second radiometer connected to the inner and outer conductorsof the diplexer and tuned to detect signals picked up from the secondantenna and produce a second output signal, and means for subtractingthe first and second output signals to produce a difference signalreflecting thermal anomalies due to the higher emissivity of thevulnerable plaques as compared to normal tissue.
 23. The apparatusdefined in claim 17 and further including a radiometer having areference input connected to the inner coaxial cable and a signal inputconnected to the inner and outer conductors of the diplexer so that whenthe catheter is moved along said vessel, said radiometer produces anoutput signal which reflects the locations of said vulnerable plaquesdue to the higher emissivity of said plaques as compared to normaltissue.
 24. Apparatus for detecting vulnerable plaques, said apparatuscomprising an intravascular catheter containing first and secondantennas; a radiometer including a Dicke switch having a reference inputand a signal input; a first cable connecting the first antenna to thereference input, and a second cable connecting the second antenna to thesignal input.
 25. The apparatus defined in claim 24 wherein said firstand second antennas are coaxial and slidable relatively to vary thespacing of the antennas.
 26. Apparatus for detecting vulnerable plaques,said apparatus comprising an intravascular catheter having proximal anddistal ends; a diplexer at the proximal end of the catheter saiddiplexer including a radially outer tubular conductor having a tubularside branch, and a radially inner tubular conductor spaced from theouter conductor and having a side branch extending through the sidebranch of the outer conductor to the outside to form a first port, saidinner and outer conductors having proximal ends which are shortcircuited to form a quarter wave stub between said proximal ends and theside branch of the inner conductor; a first antenna at the distal end ofthe catheter, said first antenna including radially inner and outerelectrically isolated tubular conductors connected electrically to therespective inner and outer conductors of the diplexer, and a secondantenna at the distal end of the catheter beyond the first antenna, saidsecond antenna including a coaxial cable which extends through the innerconductor of the diplexer to the outside to form a second port.
 27. Theapparatus defined in claim 26 wherein the coaxial cable is slidablewithin the inner conductors of the first antenna and diplexer to adjustthe distance between the first and second antennas.
 28. The apparatusdefined in claim 26 wherein the inner conductor of the coaxial cable isan open-ended tube adapted to slidably receive a guide wire.
 29. Theapparatus defined in claim 26 and further including a first radiometerconnected to the first port, and a second radiometer connected to thesecond port, said first and second radiometers having differentoperating frequencies.
 30. The apparatus defined in claim 26 and furtherincluding a radiometer having a signal input connected to the firstport, a reference input connected to the second part, and an output forconnection to a display.
 31. The apparatus defined in any one of claims1, 12, 24 or 26, wherein said catheter also includes a radiallyexpandable perforate stand-off device, and means operableextracorporeally for expanding said device.
 32. The apparatus defined inany one of claims 1, 12, 24 or 29, and further including a microwavetransmitter for transmitting a heating signal capable of ablatingtissue, and means including an additional diplexer for coupling theheating signal to the antenna but not to the radiometer(s) so that theapparatus can heat said vulnerable plaques after the detection thereof.33. A method of detecting vulnerable plaques engrained or embedded inthe wall of a patient's blood vessel, said method comprising the stepsof forming an intravascular catheter containing a microwave antenna;forming a radiometer having a signal input, a reference input and anoutput; connecting the antenna to the signal input; applying anindication of the patient's normal tissue temperature to the referenceinput, and moving the catheter along said vessel so that the locationsof the vulnerable plaques are reflected in a signal from said outputrepresenting thermal anomalies due to the higher emissivity of saidvulnerable plaques as compared to the normal tissue.
 34. The methoddefined in claim 33 and including the step of using said signal tocontrol a display device to display the locations of said vulnerableplaques in said blood vessel.
 35. The method defined in claim 33 whereinthe applying step is accomplished by providing heat sensing means in thecatheter and electrically connecting the heat sensing means to thereference input.
 36. The method defined in claim 35 including providingthe heat sensing means as a second antenna in the catheter, said firstand second antennas being coaxial.